Earth Structure:
The lithosphere is the crust and the rigid upper section of the mantle (80-90km thick). It is
divided into 7 large tectonic plates.
The asthenosphere is a semi-molten layer in the mantle where rocks are easily deformed.
Several 100s of kms thick, with tectonic plates floating on the top.
Tectonic plates move due to convection currents in the asthenosphere caused by heat from
the core due to radioactive decay and residual heat. Magma circulates as rising plumes
diverge and falling converge. Plates are hottest nearest the mid-oceanic ridges and cooler as
they move away which increases density, causing molten rock to sink down.
Continental drift and the theory of plate tectonics:
Plate tectonic theory is the modern version of Alfred Wegener’s continental drift (1912). He
proposed that 250 million years ago, in the carboniferous period, a large tectonic plate
called Pangea existed. Initially it broke apart in two land masses- north and south – that
continued to drift to form the current land masses.
Evidence of sea-floor spreading:
Theory proposed in the 1960s as magnetic field data showed that fresh molten rock from
the asthenosphere reached the sea bed and older rock was pushed away from the ridge.
Palaeomagnetism – Studying the record of the Earth’s sediment and rocks in relation to
their magnetic field. When rocks solidify magnetic fields cause minerals to align in one
direction.
The age of sea-floor rocks – In the 1960s ocean drilling showed that the thickest and oldest
sediments were near the continents and younger deposits were further out in the oceans.
Evidence from ancient glaciations – Leaves evidence called striations and deposits. Deposits
in Africa and S.America cannot have formed near the equator as there are no oceans.
Fossil Records – Many examples of similar fossils found on separate continents. When the
continents of the Southern Hemisphere are re-assembled into a single land mass, the
distribution of the fossil types forms a more realistic continuous pattern of distribution.
There are distinctive features and processes at plate boundaries:
Convergent plate boundaries:
• Continental and oceanic
Denser oceanic plate subducted under lighter continental to from ocean trench on sea floor.
Melted material is less dense and rises through weaknesses to form intrusive igneous rocks
or reaches surface and erupts as a volcano- volcanic arc. Continental crust buckles to form
Fold Mountains. The angle of subduction on the oceanic plate is 30-70 degrees; faulting
occurs in the Benioff zone, releasing energy as earthquakes.
E.g – Nazca (oceanic) and South American (continental) formed Andes Mountains, 7000km
long.
• Continental and continental
Neither subducted as low density. Two continents initially separated by an ocean floor that’s
being subducted. Eventually, the continents collide, forming fold mountains
E.g – India and Asia, Himalayas.
• Oceanic and oceanic
Subduction occurs on denser plate, forming a deep ocean trench. Non-subducted plate
forms volcanic island arc.
E.g – North American and Caribbean, Antilles volcanic arc, 17 active volcanoes including
Soufriere Hills
Divergent plate boundaries:
• Spreading Ocean Ridges
Magma rises through the asthenosphere to surface when plates move apart. This causes
new lithosphere to form where the crust is extended, thinned and fractured as magma
erupts onto the sea bed where its rapidly cooled (pillow lavas). Underwater rift valleys may
form.
Common along Mid-ocean ridges (average depth 2.5km). Long mountain chains broken up
by transform faults where earthquakes are common
E.g – South and North American plates, Mid Atlantic Ridge, rate 2-3cm per year, 10,000
miles long.
• Continental Rift Zones
Continental crust must thin considerably for rifting to occur on land. Crust being stretched
can cause faulting which produces a graben (a sunken valley). As the rift widens magma
erupts.
E.g – African and Arabian plates, Red Sea formed when graben sank. Faults cause
earthquakes.
Conservative plate boundaries:
Where plates slide past each other in a shearing motion. Frictional resistance causes
pressure which can be released as earthquakes but no volcanic activity. Escarpments mark
fault location of tension and compression but can be weathered or disappear under
accumulated sediment.
E.g – North American and Pacific Plate both moving North-West, but at different speed. On
the south-west coast of the USA (including California which receives 2-3 magnitude 5.5+
earthquakes per year), San Andreas Fault System.